Have you tested out the Japanese synthetic vocal creation program Vocaloid? Using midi notes you type in words on whatever notes and it'll sing the words in that piano key. Can adjust the vibrato at the end of notes. Usually tracks you'll hear the vibrato is a bit too heavy. They need to tone it down.

The few dozen voices are mainly Japanese but there was a couple female and one english speaking dude. If you try to make them sing words they have a heavy accent. Doesn't exactly sound American. The dude was bad but one of the females was better than the other.

I don't want to limit my tracks to english only countries so tested out having them harmonize saying ouhh, aahh, ohhh instead of words.

One track had a few layers of them replacing the usual harpsichord of some Bach. The other tune was the couple wedding songs in one. Here comes the bride and the exit song. Usually that's corny organ with no drums. I had her harmonizing the melody notes instead. Then had guitar and bass. For effect on the vocals, so it's not bland I ran it through Altiverb's convolution reverb. Those couple tracks sounded interesting. But just the bland output of Vocaloid is pretty rough. When you add the reverb it smooths out some of the detectable roughness between things and sounds more like a human female.

If you can't find someone to sing, maybe test that program out, but it's rough. You'd need to add effects so it sounds more normal.